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8/11/2009

Over the next few posts, The Rushed Behind will enlighten you as to the science of cricket. How do the different types of swing work? What causes "drift" of a spinning ball? Why does the amount of swing differ depending on the atmospheric conditions? These, and possibly more (if I can think of more questions) will be answered over the coming posts. But first, why should you believe my explanations over those of cricketers who claim it is an unexplainable art form? Well, as you will probably be surprised to know, I actually have a degree in Aerospace Engineering. So despite my bowling being more like ex-PM John Howard than Andrew Flintoff I can explain what is going on. And hopefully, in a way that a layman can understand (which is the problem with most other sources of information on these subjects).

So, first off is conventional swing. The key to conventional swing (from here on, just swing) is the seam. Lets consider a cricket ball with an angled seam, like the one drawn by Mehta below.

The fluid (air) flow over the top of the ball (as seen in the picture above) is disturbed by the seam, causing the boundary layer (fluid near the surface of the ball) to become turbulent. At this point, you are probably wondering what turbulent means. Any flow can be either laminar, turbulent or in transition between the two states. Laminar flow is nice and steady while turbulent flow has vortices and rotation of fluid. However, turbulent flow can stay attached for longer (because of the irregularities in the flow, but you don't need to know that) so the flow over the top remains attached for longer. This means that there is separation of the boundary layer earlier on the bottom of the ball and so the fluid exerts more pressure on the ball here, without the effect of the boundary layer. So there is more pressure on the bottom than the top, and the ball is moved by this pressure, towards the top.

However, the point of separation will also depend on the speed the ball is travelling at, and the smoothness of the front side. If the speed is too high or the ball is too rough then the boundary layer on the smooth side will transition towards turbulence, negating the advantage of the seam kicking the top flow into turbulence. This will negate the conventional swing, causing a straight ball and the bowler to look silly if he planned on swinging it. This is what happens when the ball stops swinging after a while as it gets roughed up. The ball may not swing from the start, as the lacquer is often less smooth than the leather of the ball itself. This will decrease the transition speed of the bottom side and so the ball will not swing. This is what is happening with the Duke balls used in England in the Ashes at the moment, with the ball starting to swing once the lacquer comes off. The wear on the ball will also lessen the effect of the seam, reducing the protuberance and potentially keeping the flow laminar for longer, reducing swing.

There are situations where conventional swing may only happen after the ball has bounced, if the seam is still in a good position and the bounce of the ball has reduced the speed below the threshold speed, but the seam position is so critical that this is rare.